DE-C2 31 49 581 discloses a two-stage combustion chamber and a method of operating it. Swirl bowls having central fuel injection nozzles are used as primary burners of the premixing type of construction. The combustion chamber is a so-called "rich/lean two-stage combustion chamber", the gases in the first combustion stage having a fuel/air equivalent ratio which is greater than 1. In the second combustion stage the gases have a fuel/air equivalent ratio which is less than 1. The transition from the rich to the lean mixture is to be realized as quickly as possible. Therefore the mixture is accelerated, and the secondary combustion air is injected into the accelerated mixture. The purpose of the acceleration is that the retention time of the mixture in the zone in which the fuel/air equivalent ratio is 1 is to be kept as short as possible. This is so, since the speed at which NO.sub.X forms is greatest at these average ratios.
Modern burners of the premixing type of construction offer the possibility of also operating the first combustion stage on a lean mixture, which has an advantageous effect on the NO.sub.X formation on account of the large air coefficient and the low flame temperatures. In such a premixing combustion technique it only has to be ensured that the flame stability, in particular at partial load, does not border on the extinction limit. It is considered to be a rule that such premixing burners, if they are operated in a single-stage manner and if temperatures of 1800 K (about 1530.degree. C.) are demanded, produce about 25-30 ppm NO.sub.X.